MRI magnet systems typically include a cylindrical vacuum vessel having a bore for receiving the subject of the imaging. A cryostat or cryogenic containment vessel inside this vacuum vessel houses the superconductive coil which provides the high strength, substantially homogeneous magnetic field necessary for the imaging. The cryostat also contains liquid helium for maintaining the coil below its critical temperature. Also there is at least one thermal radiation shield between the cryostat and the vacuum vessel. Gradient coils, which are disposed inside the bore, are pulsed and operate to determine spatial position in the imaging process. The first wall of the vacuum vessel (the cylindrical wall directly defining the bore and providing a vacuum/atmosphere interface) should be substantially "transparent" to signals generated by the gradient coils so that the induced eddy currents decay quickly.
One of the functions of the intermediate shield is to stop these signals so they do not reach the cryostat where they would generate eddy currents resulting in heating of the helium cryostat thus increasing the consumption of expensive liquid helium. However, the eddy current generation at the thermal radiation shield requires a compensation in the pulsing of the gradient coils. Since the resistivity of the material of the shield is a function of temperature and affects the generation of eddy currents, it is desirable to maintain the temperature of the shield constant so as to maintain a constant eddy current compensation.
It is known to cool the thermal radiation shield with liquid nitrogen. However, the boiling of the liquid nitrogen requires frequent addition to the magnet system. It has also been proposed to connect the cold head of a cryogenic refrigerator to the thermal radiation shield through conduction. However the output of the refrigerator can vary affecting the temperature of the shield. For further information regarding the structure and operation of this cooling apparatus, reference may be made to U.S. Pat. No. 4,537,033.
Another cooling apparatus for an MRI magnet system includes a two-stage cryogenic refrigerator mounted on the vacuum vessel in which the first stage cools a liquid nitrogen tank with liquid nitrogen supplied to cool the outer radiation shield. The colder second stage cools an inner radiation shield. Yet another cooling apparatus includes a single stage cryogenic refrigerator physically spaced from the vacuum vessel for liquefying neon and returning it to a tank inside the vacuum vessel. The tank is part of a thermal siphon for cooling the shield. For further information concerning such cooling equipment for MRI magnet systems, reference may be made to U.S. Pat. Nos. 4,535,595 and 4,680,936, respectively.